Trait mismatch across crates

1 
error[E0277]: the trait bound `rand_xorshift::XorShiftRng: rand_core::SeedableRng` is not satisfied

This is refering to XorShiftRng from the newly-published rand_xorshift crate. It does implement SeedableRng, but apparently not the same one?

I hit this error recently: because I had multiple, incompatible versions of rand_core (which provides SeedableRng) in my tree. But I have since published a compatibility shim, and in this case there is in fact only one version of rand_core in the tree (according to Cargo.lock, only version 0.3.0 is used).

What else can cause this error? And what tools are available to diagnose it?

2

There are at least two avenues to explore:

  1. You can use cargo tree to print the dependency tree of your crate.

  2. You can run cargo build --verbose and manually inspect all the crate paths that are being passed on the command line.

3

cargo tree shows only a single version of rand_core (and no other crate that should define SeedableRng).

That said, rand re-exports SeedableRng, but I'm nearly certain this wouldn't cause a problem: pub use rand_core::{RngCore, CryptoRng, SeedableRng};.

I checked the build command; it only shows a single instance of rand_core:

rustc --crate-name rand src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="alloc"' --cfg 'feature="cloudabi"' --cfg 'feature="default"' --cfg 'feature="fuchsia-zircon"' --cfg 'feature="libc"' --cfg 'feature="rand_core"' --cfg 'feature="std"' --cfg 'feature="winapi"' -C metadata=7e3bdd302bfc2846 -C extra-filename=-7e3bdd302bfc2846 --out-dir /home/dhardy/projects/rand/rand/target/debug/deps -C incremental=/home/dhardy/projects/rand/rand/target/debug/incremental -L dependency=/home/dhardy/projects/rand/rand/target/debug/deps --extern libc=/home/dhardy/projects/rand/rand/target/debug/deps/liblibc-4352b4e227216509.rlib --extern rand_chacha=/home/dhardy/projects/rand/rand/target/debug/deps/librand_chacha-d48461c6797c7159.rlib --extern rand_core=/home/dhardy/projects/rand/rand/target/debug/deps/librand_core-3d01d6cd88157b95.rlib --extern rand_hc128=/home/dhardy/projects/rand/rand/target/debug/deps/librand_hc128-aea679b9d1ad3581.rlib --extern rand_isaac=/home/dhardy/projects/rand/rand/target/debug/deps/librand_isaac-e883d9eeff2b78b7.rlib --extern rand_pcg=/home/dhardy/projects/rand/rand/target/debug/deps/librand_pcg-9de83fb8ccc92d78.rlib --extern rand_xorshift=/home/dhardy/projects/rand/rand/target/debug/deps/librand_xorshift-296e9cc137479965.rlib --cfg rust_1_26 --cfg rust_1_27
4

But what is the build command for rand_xorshift?

5

Ah, I see; yes, I see a different version of rand_core used. Would this be because of different feature flags maybe? No, it's because one of the crates depends on rand_core via a path, the other via a released crate.

Thanks for the hints.

6

You should be able to see this in cargo tree, I think?